1. Field of the Invention
The present invention relates to a linear light source having a light guide with the tapered saw tooth structures; in particular, the present invention relates to a linear light source which generates an illumination distribution and produces the complementary effect with the pickup lens and the CCD image sensor so as to be applied to high-resolution fax machines, printers and scanners.
2. Description of Related Art
At present, image read devices such as scanners, fax machines, multi-functional office apparatus and photocopy machines have been widely applied to document storage and transfer operations, and one of the cores in such devices is the optical engine module 100, as shown in FIG. 1, comprising a light source 1, a set of reflection mirrors 140, a pickup lens 120 and an image sensor 130. When light is emitted from the light source 1 onto the object (e.g., document 55 in FIG. 1), it is reflected by the object (document 55) and, through multiple reflections in the set of reflection mirrors 140, focused by the pickup lens 120 and then imaged on the image sensor 130. The created image is next converted into electronic signals by the image sensor 130. Herein the Charge Coupling Device (CCD) is commonly used as the image sensor 130, so the optical engine module is usually referred as the CCD Module (CCDM) 100.
The light source 1 is conventionally a Cold Cathode Fluorescent Lamp (CCFL). However, because of requirement on additional voltage transformers and inconsistency against environment protection regulations due to inclusion of halogen, it has be gradually replaced by Light Emitting Diodes (LEDs); for example, US Patent US2004/0095620 disclosed a light source using the LED for scanners. The LED provides advantages of power saving and long lifespan; whereas the lambert distribution (said illumination distribution in general) of the light source using the LED for scanners is less uniform than the CCFL, causing reduced scanning resolution or uneven reflection light from the document. As a result, it can be only applied in low-speed scanners.
Typically the light emitted from the light source of the CCDM should be approximately the same length as the width of the document; for example, Japan Patent JP11-232912 proposes using LEDs as the light source and an LED array is arranged at rear side of various rod lenses, thereby creating a uniform illumination distribution for application on scanners.
To achieve the desirable uniformity in the light emitted from light sources, Taiwan Patent TW579640 and US Patent US2005/0088705 disclosed different light converging devices for scanners, which essentially use the light converging components and light guiding components in order to guide the light emitted from light sources to the document to be scanned. Furthermore, the disclosure of Japan Patent JP2003262735 utilized a saw-toothed reflective surface to apply to the light guide plate in the backlight module; Taiwan Patent TW I245864 disclosed a light source having a reflective surface of saw tooth shape which includes a light guide bar including a saw-tooth-shaped reflective surface and an arc-shaped ejective surface, thereby allowing to achieve the uniformity in light emitted from the LED array by means of the light guide. US2009/0015884 and US2009/0015886 proposed installations of light source at the lateral surface of the light guide and multiple reflective recesses on the reflective surface of the light guide so as to form a uniform lambert distribution.
However, due to the effect of the optical surfaces of the pickup lens, the reflected light from document is incident into the pickup lens and focused by the pickup lens, light intensity ejected around the peripheral of the pickup lens becomes weaker relative to the center of the pickup lens. The light intensity received around the peripheral of CCD is caused as lower than the center of CCD. The uniform illumination in the sub scanning direction of the light source may thus result in less brightness at the edge of document images, and resolution of the scanner is undesirably reduced. As shown in FIG. 2, a diagram of illumination distribution and structure of a CCFL (or xenon lamp) used in a high-speed high-resolution CCDM is shown. It illustrates that the lamp electrode 72 drives the xenon lamp tube 71 to illuminate, and through specific length and illumination arrangement, the illumination distribution in the sub scanning direction forms a U-shaped illumination distribution 73. Which the U-shaped illumination distribution 73 is characterized in lower illumination at the light source center 74 but higher on both ends, thus may compensate the weaker light intensity on the peripheral of CCD such that the CCDM is allowed to generate image signals of uniform intensity. Alternatively, as shown in FIG. 3, US Patent US2005/019452 proposed the arrangement of an LED array 91 on the rear side of the rod lens 92 which controls the brightness of each LED by means of the LED array 91 thereon, such that the illumination of emitted light projected on the document to be scanned is allowed to form an even or a U-shaped illumination distribution, thereby improving the uniformity in the sub scanning direction for replacing the CCFL applied in high-speed scanners. But such a technical approach applying installation of LED array and the like on the rear side of light guide may require multiple LED elements and complicated control operations. Another technical solution is to place the light source on the front side of the light guide, as shown in FIG. 4, in which an end cap 81 is installed on the light source ejective end in order to prevent escape of significant amount of light from the adjacent side, and the reflective tooth 821 is installed on the polygonal light guide 82, such that light emitted from the light source can be sufficiently reflected in order to enhance the usability of light and the shape of light is also allowed to be modified by means of different tooth shapes and tooth pitches. Whereas, different tooth shape and tooth pitch may cause difficulties in manufacture processes, leading to undesirable high fabrication costs.
Consequently, in order to simplify manufacture processes and to reduce fabrication costs, efforts in installation of LED light source on the light guide surface so as to exhaust the LED usage for achieving more concise efficiency and to be forming a U-shaped illumination distribution complementary to the document image in the sub scanning direction on the ejective surface as well as forming a linearly converged illumination distribution in the main scanning direction, allowing to successfully accomplish the objectives of applications in high-resolution and high-speed scanning operations.